Trench process and structure for backside contact solar cells with polysilicon doped regions

ABSTRACT

A solar cell includes polysilicon P-type and N-type doped regions on a backside of a substrate, such as a silicon wafer. An interrupted trench structure separates the P-type doped region from the N-type doped region in some locations but allows the P-type doped region and the N-type doped region to touch in other locations. Each of the P-type and N-type doped regions may be formed over a thin dielectric layer. Among other advantages, the resulting solar cell structure allows for increased efficiency while having a relatively low reverse breakdown voltage.

REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional Application No.61/060,921, filed on Jun. 12, 2008.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

The invention described herein was made with Governmental support undercontract number DE-FC36-07G017043 awarded by the United StatesDepartment of Energy. The Government may have certain rights in theinvention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to solar cells, and moreparticularly but not exclusively to solar cell fabrication processes andstructures.

2. Description of the Background Art

Solar cells are well known devices for converting solar radiation toelectrical energy. They may be fabricated on a semiconductor wafer usingsemiconductor processing technology. A solar cell includes P-type andN-type doped regions. Solar radiation impinging on the solar cellcreates electrons and holes that migrate to the doped regions. In abackside contact solar cell, both the doped regions and theinterdigitated metal contact fingers coupled to them are on the backsideof the solar cell. The contact fingers allow an external electricalcircuit to be coupled to and be powered by the solar cell.

Efficiency is an important characteristic of a solar cell as it isdirectly related to the solar cell's capability to generate power.Accordingly, techniques for increasing the efficiency of solar cells aregenerally desirable. The present invention allows for increased solarcell efficiency by providing processes for fabricating novel solar cellstructures.

SUMMARY

In one embodiment, a solar cell includes polysilicon P-type and N-typedoped regions on a backside of a substrate, such as a silicon wafer. Aninterrupted trench structure separates the P-type doped region from theN-type doped region in some locations but allows the P-type doped regionand the N-type doped region to touch in other locations. Each of theP-type and N-type doped regions may be formed over a thin dielectriclayer. Among other advantages, the resulting solar cell structure allowsfor increased efficiency while having a relatively low reverse breakdownvoltage.

These and other features of the present invention will be readilyapparent to persons of ordinary skill in the art upon reading theentirety of this disclosure, which includes the accompanying drawingsand claims.

DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 show solar cell structures in accordance with anembodiment of the present invention.

FIGS. 3, 4, 5, 6, 7A, 8A, 7B, 8B, 9 and 10 illustrate the fabrication ofa solar cell in accordance with an embodiment of the present invention.

FIG. 11 shows dark I-V curves comparing the performance of a teststructure solar cell to a solar cell that is in accordance with anembodiment of the present invention.

FIG. 12 shows a flow diagram of a method of fabricating a solar cell inaccordance with an embodiment of the present invention.

FIG. 13 shows I-V curves illustrating the behavior of various solarcells as discovered by the inventors.

FIG. 14 schematically shows a solar cell in accordance with anembodiment of the present invention.

FIG. 15 schematically shows a magnified view of a portion of a backsideof the solar cell of FIG. 14 in accordance with an embodiment of thepresent invention.

FIG. 16 schematically shows a plan view of an area shown in FIG. 15.

FIG. 17 shows a cross-section of the solar cell of FIG. 14 in accordancewith an embodiment of the present invention.

FIG. 18 schematically shows a plan view of another area shown in FIG.17.

FIG. 19 shows a cross-section of the solar cell of FIG. 14 in accordancewith an embodiment of the present invention.

FIG. 20 schematically shows a plan view of an interrupted trench inaccordance with an embodiment of the present invention.

FIG. 21 shows I-V curves representing reverse breakdown characteristicsof various solar cells in experiments performed by the inventors.

FIG. 22 shows a flow diagram of a method of fabricating a solar cell inaccordance with an embodiment of the present invention.

FIG. 23 schematically shows a solar cell with a single trenchinterruption in accordance with an embodiment of the present invention.

FIG. 24 schematically shows a plan view of an interrupted trench in thesolar cell of FIG. 23.

FIG. 25 schematically shows a solar cell with multiple trenchinterruptions in a single region having substantially the same localoperating temperature in accordance with an embodiment of the presentinvention.

FIG. 26 schematically shows a plan view of an interrupted trench in thesolar cell of FIG. 25.

The use of the same reference label in different figures indicates thesame or like components. The figures are not drawn to scale.

DETAILED DESCRIPTION

In the present disclosure, numerous specific details are provided, suchas examples of materials, process parameters, process steps, andstructures, to provide a thorough understanding of embodiments of theinvention. Persons of ordinary skill in the art will recognize, however,that the invention can be practiced without one or more of the specificdetails. In other instances, well-known details are not shown ordescribed to avoid obscuring aspects of the invention.

In solar cells with P-type and N-type doped regions in the substrate,the P-type and N-type doped regions may be formed with separate orabutting perimeters. However, this is not true with polysilicon dopedregions because recombination in the space charge region where thepolysilicon doped regions touch is very high due to the lifetime ofcharge carriers in the polysilicon being very low. That is, the touchingpolysilicon doped regions adversely affect efficiency. Embodiments ofthe present invention address this problem associated with polysilicondoped regions and formed doped regions in general.

FIG. 1 schematically shows a sectional view of a solar cell structure inaccordance with an embodiment of the present invention. In the exampleof FIG. 1, the solar cell is a backside contact solar cell in that itsdoped regions 101 and 102 are on the backside 106 opposite to the frontside 105. The front side 105 faces the sun during normal operation. Thedoped regions 101 and 102 are formed on a thin dielectric layer 113. Thedielectric layer 113 may be formed to a thickness of 5 Angstroms to 40Angstroms. In one embodiment, the dielectric layer 113 comprises silicondioxide thermally grown on the surface of the substrate 103 to athickness of 20 Angstroms. The dielectric layer 113 may also comprisesilicon nitride. The dielectric layer 113 advantageously allows forsurface passivation. The polysilicon of the doped regions 101 and 102applies an electric field across the dielectric layer 113, which repelsminority carriers and accumulates majority carriers at the dielectricinterface.

In the example of FIG. 1, the doped region 101 is a P-type doped region,while the doped region 102 is an N-type doped region. A substrate 103comprises an N-type silicon wafer in this example. As can beappreciated, the substrate 103 may also comprise a P-type silicon orother wafer with appropriate changes to the rest of the structure. Thereare several P-type and N-type doped regions in any given solar cell butonly one of each is shown in FIG. 1 for clarity of illustration.

The doped regions 101 and 102 may comprise doped polysilicon formed to athickness of about 2000 Angstroms by low pressure chemical vapordeposition (LPCVD). The doped region 101 may comprise polysilicon dopedwith a P-type dopant (e.g., boron) and the doped region 102 may comprisepolysilicon doped with an N-type dopant (e.g., phosphorus). Thepolysilicon may be deposited over the thin dielectric layer 113 and thendoped by diffusion. The polysilicon may also be pre-doped prior todeposition on the dielectric layer 113. Polysilicon is the preferredmaterial for the doped regions 101 and 102 for its compatibility withhigh temperature processing, allowing for increased thermal budget.

As shown in FIG. 1, the doped regions 101 and 102 are separated by atrench 104, which serves as a gap between the doped regions 101 and 102.

The trench 104 may be formed by laser trenching or conventional etching,for example. In one embodiment, the trench 104 is about 100 micronswide. The trench 104 may be formed before or after a diffusion step thatdopes the polysilicon doped regions 101 and 102. If the trench 104 isformed before the diffusion step, the passivation region 112 maycomprise an N-type passivation region formed during the diffusion step.

In one embodiment, the trench 104 is formed using a process that notonly forms the trench 104 but also forms a randomly textured surface 114on the surface of the trench 104. The randomly textured surface 114improves solar radiation collection of light incident on the back of thesolar cell, i.e. a bifacial configuration. A wet etch process comprisingpotassium hydroxide and isopropyl alcohol may be used to form the trench104 and to texture the surface 114 with random pyramids. The trench 104may be formed to dig 1 to 10 microns (e.g., 3 microns) into thesubstrate 103.

A dielectric in the form of a silicon nitride 107 is deposited in thetrench 104. The silicon nitride 107 preferably has a relatively largepositive fixed charge density to place the silicon surface under thetrench 104 in accumulation and to provide good surface passivation. Thepositive fixed charge density of the silicon nitride 107 may naturallyoccur as part of the deposition process used to form the silicon nitride107. In one embodiment, the silicon nitride 107 is formed to a thicknessof about 400 Angstroms by plasma enhanced chemical vapor deposition(PECVD). The resulting accumulation layer repels minority carriers, i.e.positively charged holes in N-type material. The trench 104 alsoprevents the space charge region from developing in the polysilicon.Instead, the space charge develops in the single crystal siliconunderneath the P-type polysilicon. In this region, lifetime is notreduced due to grain boundaries, and hence the parasitic recombinationis suppressed. A portion of this space charge region also intersects thesurface of the wafer in the trench 104. The positive charge in thesilicon nitride 107 reduces the impact of this region of space chargeregion as well narrowing the region.

An example process flow for fabricating the solar cell structure of FIG.1 may include forming a thin dielectric layer 113 over a backsidesurface of the substrate 103, forming an undoped polysilicon layer overthe thin dielectric layer 113, doping the polysilicon layer into P-typeand N-type doped regions 101 and 102, etching the doped polysiliconlayer to form the trench 104 and the textured surface 114, forming thepassivation region 112, and forming the silicon nitride 107 in thetrench 104. Rather than diffusing dopants on an undoped polysiliconlayer, the doped regions 101 and 102 may also be formed by depositingpre-doped polysilicon on the dielectric layer 113 using conventionaldeposition, masking, and etching techniques. The silicon nitride 107preferably has a planar, as opposed to textured, surface. However, theplanarity of the silicon nitride 107 is not critical and no additionalplanarization step is needed. For example, the planarity of the siliconnitride 107 may be as deposited. The trench 104 may be formed before orafter doping of the doped regions 101 and 102.

Referring to FIG. 2, interdigitated metal contact fingers 108 and 109may be formed through the silicon nitride 107 to make an electricalconnection to the doped regions 101 and 102, respectively. Externalelectrical circuits may be attached to the interdigitated metal contactfingers 108 and 109 to connect to and be powered by the solar cell. Inthe example of FIG. 2, the metal contact finger 108 may be connected toa positive electrical terminal and the metal contact finger 109 may beconnected to a negative electrical terminal.

The trench structure of FIG. 1 addresses the aforementioned issuesrelating to polysilicon parasitic space charge recombination severalways. Firstly, the trench 104 separates the doped regions 101 and 102 sothey are not physically in contact. This prevents the space chargeregion from existing in either polysilicon film. Secondly, the resultingaccumulation layer under the trench 104 repels minority carriers toimprove surface passivation. Thirdly, the textured surface 114 in thetrench 104 increases solar radiation collection. These advantageouslyhelp increase solar cell efficiency.

FIGS. 3-10 show sectional views illustrating the fabrication of a solarcell in accordance with an embodiment of the present invention. Thereare a plurality of P-type doped regions and N-type doped regions in asolar cell but only one of each is shown as being fabricated in thefollowing example for clarity of illustration.

The embodiment of FIGS. 3-10 begins with formation of a thin dielectriclayer 313 on a backside surface of a substrate 303 (FIG. 3). Thesubstrate 303 may comprise an N-type silicon wafer, for example. Thedielectric layer 313 may be formed to a thickness of 5 Angstroms to 40Angstroms (e.g., 20 Angstroms). In one embodiment, the dielectric layer313 comprises silicon dioxide thermally grown on the surface of thesubstrate 103. The dielectric layer 313 may also comprise siliconnitride, for example. An undoped polysilicon layer 322 is then formed onthe dielectric layer 313. The polysilicon layer 322 may be formed to athickness of about 2000 Angstroms by LPCVD, for example. A doped silicondioxide layer 323 is then formed over the polysilicon layer 322 (FIG.4). The silicon dioxide layer 323 serves as a dopant source for asubsequently formed doped region, which is a P-type doped region 301 inthis example (see FIG. 7A or 8B). The silicon dioxide layer 323 may thusbe doped with a P-type dopant, such as boron. The doped silicon dioxidelayer 323 is patterned to remain over an area of the polysilicon layer322 where the P-type doped region 301 is to be formed (FIG. 5). Thesilicon dioxide layer 323 may be formed to a thickness of about 1000Angstroms by APCVD.

A doped silicon dioxide layer 324 is formed over the silicon dioxide 323and the polysilicon layer 322 (FIG. 6). The silicon dioxide 324 servesas a dopant source for a subsequently formed doped region, which is anN-type doped region 302 in this example (see FIG. 7A or 8B). The silicondioxide 324 may thus be doped with an N-type dopant, such as phosphorus.The silicon dioxide 324 may be formed to a thickness of about 2000Angstroms by APCVD.

The trench separating the doped regions may be formed before formationof the doped regions in a first trench formation process or afterformation of the doped regions in a second trench formation process.FIGS. 7A and 8A illustrate process steps for the first trench formationprocess, while FIGS. 7B and 8B illustrate process steps for the secondtrench formation process. Both trench formation processes may proceedfrom FIG. 6 and continue on to FIG. 9.

In the first trench formation process, a thermal drive-in step diffusesdopants from the silicon dioxides 323 and 324 to the underlyingpolysilicon layer 322, thereby forming P-type and N-type doped regionsin the polysilicon layer 322, which is accordingly relabeled as P-typedoped region 301 and N-type doped region 302 (FIG. 7A). The thermaldrive-in step may be performed by heating the sample of FIG. 6. Thepreferred drive conditions give a heavily doped, e.g., greater than 1e²⁰ cm⁻³, polysilicon layer that is uniform throughout the thickness ofthe film and has very little doping under the polysilicon, e.g., equalto or less than 1 e¹⁸ cm⁻³. The thermal drive-in step results in thepolysilicon layer 322 under the silicon dioxide 323 forming the P-typedoped region 301 and polysilicon layer 322 under the silicon dioxide 324forming the N-type doped region 302.

The silicon dioxide 324, silicon dioxide 323, doped region 301, dopedregion 302, and thin dielectric layer 313 are etched to form a trench304 (FIG. 8A). The trench etch may comprise a multi-step etch process,with the last etch step stopping on the substrate 303. The trench 304may be about 100 microns wide, for example. However, there is no knownlimit to the minimum width as long as the P-type doped region 301 andN-type doped region 302 do not contact each other. The trench 304 may beformed by conventional etching processes including by laser trenching.In one embodiment, the trench 304 has a textured surface 314 forimproved solar radiation collection efficiency. In one embodiment, a wetetch process comprising potassium hydroxide and isopropyl alcohol isused to form the trench 304 and to texture the surface 314 with randompyramids. The trench 304 may extend 1 to 10 microns, e.g., 3 microns,into the substrate 303.

A thin (less than 200 Angstroms, e.g., 100 Angstroms) passivation layer310 may be formed on the surface 314 of the trench 304. The passivationlayer 310 may comprise silicon dioxide thermally grown on the surface314 or deposited silicon nitride layer, for example.

In the second trench formation process, the silicon dioxide 324, silicondioxide 322, and thin dielectric layer 313 of the sample of FIG. 6 areetched to form the trench 304 (FIG. 7B). Textured surface 314 may beformed on the surface of the trench 304. The trench etch is essentiallythe same as in the first trench formation process except that the trenchis formed before formation of the doped regions of the solar cell.

A thermal drive-in step is performed to diffuse dopants from the silicondioxide layers 323 and 324 to the underlying polysilicon layer 322,thereby forming the doped regions 301 and 302 as in the first trenchformation process (FIG. 8B). In this case, in the second trenchformation process, a passivation region 315 is formed in the substrate303 under the trench 304 during the diffusion process. The passivationregion 315 may comprise diffused N-type dopants. In one embodiment, thepassivation region 315 is formed by introducing POCl3 (phosphoruschloride oxide) in the diffusion furnace during the thermal drive-in.The passivation region 315 serves the same function as the passivationregion 112 of FIG. 1.

In both the first and second trench formation processes, the trench 304serves as a gap physically separating the P-type doped region 301 fromthe N-type doped region 302. The processing of the solar cell continuesfrom either FIG. 8A or 8B to FIG. 9.

Continuing with FIG. 9, a dielectric in the form of a silicon nitridelayer 307 is formed in the trench 304. In the example of FIG. 9, thesilicon nitride layer 307 is also formed over the layers 323 and 324.The silicon nitride layer 307 preferably has a relatively large positivefixed charge density to place the silicon surface under the trench 304in accumulation and to provide good surface passivation. The positivefixed charge density on the silicon nitride layer 307 may naturallyoccur as part of a PECVD process, for example. In one embodiment, thesilicon nitride 307 is formed to a thickness of about 400 Angstroms byPECVD. The silicon nitride 307 preferably has a planar (e.g., asdeposited) surface. In FIGS. 9 and 10, the passivation region 312represents either the passivation layer 310 (see FIG. 8A) or thepassivation region 315 (see FIG. 8B) depending on the trench formationprocess used.

Interdigitated metal contact fingers 308 and 309 may then be formedthrough the silicon nitride 307 to make an electrical connection to thedoped regions 301 and 302 by way of layers 323 and 324, respectively(FIG. 10). External electrical circuits may be attached to theinterdigitated metal contact fingers 308 and 309 to connect to and bepowered by the solar cell. In the example of FIG. 10, the metal contactfinger 308 may be coupled to a positive electrical terminal and themetal contact finger 309 may be coupled to a negative electricalterminal. The resulting solar cell provides the same benefits as thesolar cell of FIG. 1.

FIG. 11 shows dark I-V (i.e., current-voltage) curves comparing theperformance of a conventional solar cell to a solar cell that is inaccordance with an embodiment of the present invention. The I-V curvesare “dark” in that they were measured with no direct solar radiationshining on the solar cells.

The I-V curves are for the diodes formed between an N-type silicon and aP-type doped region. In the example of FIG. 11, the horizontal axisrepresents voltage across the diode and the vertical axis represents theresulting current across the diode. Plot 401 is the I-V curve for a teststructure solar cell with touching P-type and N-type polysilicon dopedregions, plot 402 is the I-V curve for a typical Sunpower CorporationA300™ solar cell, and plot 403 is for a solar cell having a trenchbetween the P-type and N-type doped regions as in FIGS. 1 and 9. Whilethe plot 402 is very close to the ideal I-V curve represented by theplot 404, the plot 403 is even closer. The plot 405 represents a guidefor the eye of an ideal diode I-V characteristic, the slope of which is60 millivolts per decade of current.

Referring now to FIG. 12, there is shown a flow diagram of a method 600of fabricating a solar cell in accordance with an embodiment of thepresent invention. In the method 600, doped regions are formed in apolysilicon layer (step 601). The doped regions may be formed bydepositing doped silicon dioxide layers over an undoped polysiliconlayer and performing a diffusion step, by depositing pre-doped silicondioxide layers, or by depositing an undoped polysilicon layer followedby a dopant implantation step, for example. The polysilicon layer wherethe doped regions are formed may be etched to form a trench separatingthe P-type doped region from the N-type doped region (step 602).Alternatively, the trench is formed before the doped regions are formed.The trench may include a textured surface for increased solar radiationcollection. A passivation region, such as passivation layer or adiffused region in the substrate, may be formed to isolate trenchmaterial from the bulk of the substrate (step 603). A dielectric in theform of a silicon nitride layer may then be deposited in the trench(step 604). Interdigitated metal contact fingers may thereafter beformed to electrically connect to the P-type and N-type doped regionsthrough the silicon nitride.

As explained, physically separating the P-type and N-type doped regionshelps increase the solar cell's efficiency by removing the buttingjunction where these doped regions touch. Although the butting junctionhas deleterious effect on the solar cell's efficiency due to resultingrelatively high forward leakage current, the butting junction lowers thereverse breakdown voltage of the solar cell. The reverse breakdownvoltage typically occurs when the solar cell is shaded (i.e., no solarradiation directly shining on the solar cell). The reverse breakdownvoltage is from avalanche and zener breakdown mechanisms in the diodeformed by the butting portions of the P-type and N-type doped regions.The reverse breakdown voltage of some solar cells with butting junctionsis about −4 volts, for example. In comparison, because it has no buttingjunction, a solar cell with fully trenched P-type and N-type polysilicondoped regions have much higher reverse breakdown voltage, which may beas high as −100 volts. A high reverse breakdown voltage may result inexcessive heat and potentially poses a safety issue.

FIG. 13 shows I-V curves illustrating the behavior of various solarcells as discovered by the inventors. In FIG. 13, the horizontal axisrepresents voltage and the vertical axis represents current. The firstquadrant I to the right of the origin (0,0) represents positive voltageand current. The fourth IV quadrant to the left of the origin (0, 0)represents negative voltage and positive current.

I-V curve 701 is for a first sample solar cell without a trenchseparating the P-type and N-type doped regions. I-V curve 702 is for asecond sample solar cell with a continuous trench separating the P-typeand N-type doped regions, such as the solar cell shown in FIG. 2.

As the first sample solar cell is shaded, the curve 701 goes to the leftas the voltage developed by the first sample solar cell becomes morenegative. The butting junction diode eventually breaks down at a reversebreakdown voltage, such as −4 volts. The resulting increased in currentis limited by other solar cells in the same module and is usually not aconcern. While the reverse breakdown voltage of the first sample solarcell is relatively low, the cell's performance in the forward voltageregion (i.e., when solar radiation is impinging on the solar cell; seethe first quadrant I) is not competitive relative to that of the secondsample solar cell. More specifically, I-V curve 702 of the second samplesolar cell indicates that the second sample solar cell is more efficientthan the first sample solar cell during normal operation, as can be seenby comparing I-V curves 701 and 702 in the first quadrant I.

The inventors discovered the unexpected result that by interrupting atrench separating the P-type and N-type doped regions at differentportions of the trench such that the P-type and N-type doped regionstouch in the interrupted portions, the reverse breakdown characteristicsof a solar cell can be improved without substantially adverselyaffecting its forward voltage characteristics during normal operation.

In FIG. 13, the I-V curve 703 is for a third sample solar cell withinterrupted trenches in accordance with an embodiment of the presentinvention. As the third sample solar cell is shaded, the curve 703 goesto the left as the voltage developed by the third sample solar cellbecomes more negative. The third sample solar cell eventually breaksdown at a reverse breakdown voltage of about −6 volts, which is a littlelarger compared to that of the second sample solar cell. However, theforward voltage characteristics of the third sample solar cell iscomparable to that of the second sample solar cell, as can be seen bycomparing I-V curves 703 and 702 in the first quadrant I. That is, theuse of interrupted trenches allows for improved efficiency during normaloperation when the sun is shining on the solar cell, while having arelatively low reverse breakdown voltage when the solar cell is shaded.A solar cell with interrupted trenches is now described beginning withFIG. 14.

FIG. 14 schematically shows a solar cell 720 in accordance with anembodiment of the present invention. The solar cell 720 may comprise abackside contact solar cell having P-type and N-type doped regions in apolysilicon layer formed on the backside 721. The front side of thesolar cell 720, which is opposite the backside 721, faces towards thesun during normal operation. In the example of FIG. 14, a plurality oftrench interruptions 723 represents interruptions or breaks ininterrupted trenches between P-type and N-type doped regions. Only someof the trench interruptions 723 have been labeled for clarity ofillustration.

FIG. 15 schematically shows a magnified view of a portion of thebackside 721 in accordance with an embodiment of the present invention.In the example of FIG. 15, metal contact fingers 108 and 109 areinterdigitated on the backside 721. An interrupted trench 800 separatesa P-type doped region 101 and an N-type doped region 102 in mostlocations. Trench interruptions 723 (only one is shown in FIG. 15)breaks continuity of the trench 800 at certain locations, allowing theP-type doped region 101 and the N-type doped region 102 to physicallytouch or abut in those locations. Metal contact fingers 108 and 109electrically couple to the P-type and N-type doped regions,respectively. In practice, as shown in FIGS. 17 and 19, the metalcontact fingers 108 and 109 connect to their respective doped regions byway of a dielectric, which in one embodiment comprises a silicon nitride107 (e.g., see FIGS. 17 and 19).

As will be more apparent below, the solar cell 720 is the same as thepreviously described solar cells having a trench (e.g., see FIGS. 2 and10) except that the trench 800 of the solar cell 720 is interrupted toallow for improved efficiency with lower reverse breakdown voltage.Accordingly, the solar cell 720 may be fabricated using the same processas previously described for the solar cells of FIGS. 2 and 10 exceptthat the trench 800 is not continuous as it is interrupted at certainlocations. The trench interruptions may be formed by using acorresponding mask pattern or by appropriate control of a laser in thetrench etch step. For example, a mask pattern for the trench etch mayinclude trench interruptions where the trench is not dug. As anotherexample, the laser digging the trench may be turned off in the trenchinterruptions.

FIG. 16 schematically shows a plan view of an area of the backside 721generally bounded by a dashed perimeter 725 in FIG. 15. FIG. 16 shows acontinuous (i.e., not interrupted) portion of the trench 800. The trench800 physically separates the P-type doped region 101 and the N-typedoped region 102. FIG. 16 also shows the P-type metal contact finger 108electrically coupled to the P-type doped region 101 and the N-type metalcontact finger 109 electrically coupled to the N-type doped region 102.

FIG. 17 shows a cross-section taken at section A-A of FIG. 16 inaccordance with an embodiment of the present invention. FIG. 17 showsthe backside 721 and the front side 722 of the solar cell. It is to benoted that the cross-section of FIG. 17 is the same as that shown inFIG. 2 except for the trench 800. The trench 104 of FIG. 2 is continuousthroughout whereas the trench 800 of FIG. 17 has interruptions. Theother structures labeled in FIG. 17 have been previously explained withreference to FIG. 2.

FIG. 18 schematically shows a plan view of an area of the backside 721generally bounded by a dashed perimeter 726 in FIG. 15. FIG. 18 shows atrench interruption 723 where the P-type doped region 101 and the N-typedoped region 102 touch. A dashed line 727 indicates the butting junctionformed by the P-type doped region 101 and the N-type doped region 102.In practice, there are several trench interruptions 723 along the lengthof the trench 800. Only one trench interruption 723 is shown in FIG. 18for clarity of illustration. FIG. 18 also shows the P-type metal contactfinger 108 electrically coupled to the P-type doped region 101 and theN-type metal contact finger 109 electrically coupled to the N-type dopedregion 102.

FIG. 19 shows a cross-section taken at section B-B of FIG. 18. FIG. 19shows the backside 721 and the front side 722 of the solar cell. It isto be noted that the cross-section of FIG. 19 is the same as that shownin FIG. 17 except without the trench 800 because the cross-section istaken at a trench interruption 723. Accordingly, the silicon nitride 107extends from the metal contact finger 108 to the metal contact finger109. Also, the P-type doped region 101 and the N-type doped region 102form a butting junction 727. Generally speaking, the P-type doped region101 and the N-type doped region 102 at the butting junction 727 form adiode with a reverse breakdown voltage that is relatively low. Thisadvantageously allows the solar cell to have relatively low reversebreakdown voltage when shaded. Higher doping concentration and higherdoping gradient of the polysilicon where the P-type doped region 101 andN-type doped region 102 are formed lower the reverse breakdown voltage.The lower reverse breakdown voltage allows the perimeter of the buttingjunction to be reduced by a factor of 100, which only increases thereverse breakdown voltage from −4 volts to −6 volts in one example. Theother structures labeled in FIG. 19 have been previously explained withreference to FIG. 2.

FIG. 20 schematically shows a plan view of an interrupted trench 800 inaccordance with an embodiment of the present invention. A plurality oftrench interruptions 723 break continuity of the trench 800, allowingfor formation of butting junctions between adjacent P-type and N-typedoped regions in the interruptions. In one embodiment, the interruptions723 constitute about 0.1% to 10% of the total length of the trench 800if the trench 800 were continuous. The total length of the trench 800may be approximately 125 mm, which is a dimension across the wafer.

FIG. 21 shows I-V curves representing reverse breakdown characteristicsof various solar cells in experiments performed by the inventors. In theexample of FIG. 21, the horizontal axis represents reverse voltage andthe vertical axis represents current. I-V curve 731 is for a teststructure solar cell without trenches and accordingly has touchingP-type and N-type polysilicon doped regions throughout, I-V curve 732 isfor a solar cell with continuous trenches as in FIGS. 2 and 10, I-Vcurve 733 is for a typical Sunpower Corporation A300™ solar cell, andI-V curve 734 is for a solar cell having interrupted trenches. Note thatthe curve 731 of the solar cell with no trench is fairly steep,resulting in a lower reverse breakdown voltage compared to the curve 732of the solar cell with full trenches. The curve 734 of the solar cellwith interrupted trenches indicates a slightly higher reverse breakdownvoltage compared to curves 731 and 733. However, as previouslyexplained, the interrupted trenches allow for higher efficiency in theforward voltage region, which is during normal operation when the solarcell is fully exposed to the sun.

FIG. 22 shows a flow diagram of a method 730 of fabricating a solar cellin accordance with an embodiment of the present invention. In the method730, P-type and N-type doped regions are formed in a polysilicon layer(step 731). The doped regions may be formed by depositing doped silicondioxide layers over an undoped polysilicon layer and performing adiffusion step, by depositing pre-doped silicon dioxide layers, or bydepositing an undoped polysilicon layer followed by a dopantimplantation step, for example. The polysilicon layer where the dopedregions are formed may be etched to form an interrupted trench betweenthe P-type doped region from the N-type doped region (step 732). Theinterrupted trench may separate the P-type and N-type doped regions atcontinuous portions (e.g., 99% of the total length) of the interruptedtrench and allows the P-type and N-type doped regions to touch atinterrupted portions (e.g., 1% of the total length) of the interruptedtrench. The interrupted trench may also be formed before the dopedregions are formed. The interrupted trench may include a texturedsurface for increased solar radiation collection. A passivation region,such as passivation layer or a diffused region in the substrate, may beformed to isolate trench material from the bulk of the substrate (step733). A dielectric in the form of a silicon nitride layer may then bedeposited in the interrupted trench (step 734). Interdigitated metalcontact fingers may thereafter be formed to electrically connect to theP-type and N-type doped regions through the silicon nitride.

Depending on design and environmental particulars, having several trenchinterruptions 723 along the trench 800 may lead to thermal runaway andoverheat the solar cell. To prevent thermal runaway, the number andplacement of trench interruptions 723 on a trench 800 may be limited.These embodiments are now explained beginning with FIG. 23.

FIG. 23 schematically shows a solar cell 750 in accordance with anembodiment of the present invention. The solar cell 750 is the same asthe solar cell 720 (see FIG. 14) except that the solar cell 750 has onlyone trench interruption 723 on the entire backside 721. That is, thereis no other trench interruption 723 on any trench of the solar cell 750.

FIG. 24 schematically shows a plan view of an interrupted trench 800 inthe solar cell 750. As before, the trench 800 physically separates theP-type doped region 101 and N-type doped region 102. The single trenchinterruption 723 breaks continuity of the trench 800, allowing forformation of a butting junction between the P-type doped region 101 andthe N-type doped region 102 in a single location in the solar cell 750.Generally speaking, there may be one to ten trench interruptions, with asingle trench interruption for the entire wafer being preferable.

FIG. 25 schematically shows a solar cell 751 in accordance with anembodiment of the present invention. The solar cell 751 is the same asthe solar cell 720 (see FIG. 14) except that the solar cell 751 hasseveral trench interruptions 723 (three in the example of FIG. 25)concentrated on only one continuous region 752. That is, there is noother region 752 (hence no other trench interruptions 723) in the solarcell 751. The entire area of the single region 752 has substantially thesame or common local operating temperature.

FIG. 26 schematically shows a plan view of an interrupted trench 800 inthe solar cell 751. As before, the trench 800 physically separates theP-type doped region 101 and the N-type doped region 102. The trenchinterruptions 723 in the single region 752 break continuity of thetrench 800, allowing for formation of butting junctions between theP-type doped region 101 and the N-type doped region 102 in the singleregion 752.

As can be appreciated from the foregoing, solar cells in accordance withembodiments of the present invention may have trench interruptions 723spaced throughout the length of a trench 800, only one trenchinterruption 723, a plurality of trench interruptions 723 on only onecontinuous region, and other design combinations (e.g., two localregions, each of which has several spaced trench interruptions 723; twotrench interruptions 723 in separate regions with substantiallydifferent local operating temperatures) without detracting from themerits of the present invention.

Improved solar cell fabrication processes and structures have beendisclosed. While specific embodiments of the present invention have beenprovided, it is to be understood that these embodiments are forillustration purposes and not limiting. Many additional embodiments willbe apparent to persons of ordinary skill in the art reading thisdisclosure.

1. A solar cell structure comprising: a doped silicon substrate having afront side configured to face the sun during normal operation and abackside opposite the front side; a P-type doped region and an N-typedoped region comprising polysilicon, wherein the P-type doped region andthe N-type doped region are physically separate in one location andtouch each other forming a butting junction in another location; a firstdielectric layer between the substrate and the P-type doped region andthe N-type doped region; and a second dielectric layer formed over theP-type doped region and the N-type doped region.
 2. The solar cellstructure of claim 1 wherein the first dielectric layer comprisessilicon dioxide formed to a thickness between 5 to 40 Angstroms on asurface of the silicon substrate.
 3. The solar cell structure of claim 1wherein the P-type doped region and the N-type doped region arephysically separated by an interrupted trench structure.
 4. The solarcell structure of claim 1 further comprising: a passivation layerbetween the second dielectric layer and the substrate.
 5. The solar cellstructure of claim 3 further comprising a diffused passivation region inthe substrate under the interrupted trench structure, wherein thepassivation region is doped with an N-type dopant.
 6. The solar cellstructure of claim 1 further comprising interdigitated metal contactfingers electrically coupled to the P-type and N-type doped regionsthrough the second dielectric layer.
 7. A solar cell comprising: aP-type doped region and an N-type doped region formed on a backside of adoped silicon substrate, the P-type doped region and the N-type dopedregion comprising polysilicon, each of the P-type and N-type dopedregions being formed over a dielectric layer that is formed between thesilicon substrate and the P-type and N-type doped regions; and aninterrupted trench structure separating the P-type doped region and theN-type doped region and dividing the dielectric layer, the interruptedtrench structure allowing the P-type doped region and the N-type dopedregion to touch in a first location.
 8. The solar cell of claim 7wherein the silicon substrate comprises an N-type silicon substrate. 9.The solar cell of claim 7 further comprising a second location where theP-type doped region and the N-type doped region touch.
 10. The solarcell of claim 9 wherein the first and second locations are in a sameregion having substantially same local operating temperature.
 11. Thesolar cell of claim 7 wherein the first location is the only location inthe solar cell where the P-type doped region and the N-type doped regiontouch.
 12. The solar cell of claim 7 further comprising a passivationlayer formed on a surface of the interrupted trench structure.